Sharath Girimaji - Academia.edu (original) (raw)

Papers by Sharath Girimaji

Research paper thumbnail of Nonlinear evolution of perturbations in high Mach number wall-bounded flow: Pressure–dilatation effects

Physics of Fluids, 2020

We characterize the nonlinear evolution of perturbations in a high Mach number Poiseuille flow an... more We characterize the nonlinear evolution of perturbations in a high Mach number Poiseuille flow and contrast the behavior against an equivalent incompressible flow. The focus is on the influence of pressure–dilatation on (i) internal energy evolution, (ii) kinetic–internal energy exchange, and (iii) kinetic energy spectrum evolution. We perform direct numerical simulations of plane Poiseuille flow at different Mach numbers subject to a variety of initial perturbations. In all high-speed cases considered, pressure dilatation leads to energy equipartition between wall-normal velocity fluctuations (dilatational kinetic energy) and pressure fluctuations (a measure of internal energy). However, the effect of pressure–dilatation on the kinetic energy spectral growth can be varied. In cases wherein pressure–dilatation is larger than the turbulent kinetic energy production, spectral growth is considerably slow relative to an equivalent low Mach number case. When pressure–dilatation is smalle...

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Research paper thumbnail of Continuum breakdown in compressible mixing layers

Physical Review E

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Research paper thumbnail of Proxy-equation' paradigm - A novel strategy for massively-parallel asynchronous computations

Massively parallel simulations of transport equation systems call for a paradigm change in algori... more Massively parallel simulations of transport equation systems call for a paradigm change in algorithm development to achieve efficient scalability. Traditional approaches require time synchronization of processing elements (PEs) which severely restricts scalability. Relaxing synchronization requirement introduces error and slows down convergence. In this paper, we propose and develop a novel `proxy-equation' concept for a general transport equation that (i) tolerates asynchrony with manageable added error, (ii) preserves convergence order and (iii) scales efficiently on massively parallel machines. The central idea is to modify a priori the transport equation at the PE boundaries to offset asynchrony errors. Proof-of-concept computations are performed using a one-dimensional advection-diffusion equation. The results demonstrate the promise and advantages of the present strategy.

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Research paper thumbnail of Magneto-Gas Kinetic Method for Nonideal

In this work, the gas-kinetic method (GKM) is enhanced with resistive and Hall magnetohy-drodynam... more In this work, the gas-kinetic method (GKM) is enhanced with resistive and Hall magnetohy-drodynamics (MHD) effects. Known as MGKM (for MHD–GKM), this approach incorporates additional source terms to the momentum and energy conservation equations and solves the magnetic field induction equation. We establish a verification protocol involving numerical solutions to the one-dimensional (1D) shock tube problem and two-dimensional (2D) channel flows. The contributions of ideal, resistive, and Hall effects are examined in isolation and in combination against available analytical and computational results. We also simulate the evolution of a laminar MHD jet subject to an externally applied magnetic field. This configuration is of much importance in the field of plasma pro-pulsion. Results support previous theoretical predictions of jet stretching due to magnetic field influence and azimuthal rotation due to the Hall effect. In summary, MGKM is estab-lished as a promising tool for investiga...

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Research paper thumbnail of Towards Reconstruction of Complex Flow Fields Using Unit Flows

Fluids, 2021

Many complex turbulent flows in nature and engineering can be qualitatively regarded as being con... more Many complex turbulent flows in nature and engineering can be qualitatively regarded as being constituted of multiple simpler unit flows. The objective of this work is to characterize the coherent structures in such complex flows as a combination of constituent unitary flow structures for the purpose of reduced-order representation. While turbulence is clearly a non-linear phenomenon, we aim to establish the degree to which the optimally weighted superposition of unitary flow structures can represent the complex flow structures. The rationale for investigating such superposition stems from the fact that the large-scale coherent structures are generated by underlying flow instabilities that may be reasonably described using linear analysis. Clearly, the degree of validity of superposition will depend on the flow under consideration. In this work, we take the first step toward establishing a procedure for investigating superposition. Experimental data of single and triple tandem jets ...

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Research paper thumbnail of The Prospect and Benefits of Using the Partial-Averaged Navier-Stokes Method for Engine Flows

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Research paper thumbnail of Manufactured Turbulence with Langevin equations

By definition, Manufactured turbulence(MT) is purported to mimic physical turbulence rather than ... more By definition, Manufactured turbulence(MT) is purported to mimic physical turbulence rather than model it. The MT equations are constrained to be simple to solve and provide an inexpensive surrogate to Navier-Stokes based Direct Numerical Simulations (DNS) for use in engineering applications or theoretical analyses. In this article, we investigate one approach in which the linear inviscid aspects of MT are derived from a linear approximation of the Navier-Stokes equations while the non-linear and viscous physics are approximated via stochastic modeling. The ensuing Langevin MT equations are used to compute planar, quadratic turbulent flows. While much work needs to be done, the preliminary results appear promising.

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Research paper thumbnail of Characterization of coherent structures in turbulent wake of a sphere using partially averaged Navier–Stokes (PANS) simulations

Physics of Fluids, 2020

The large-scale coherent structures in the wake of a sphere at subcritical Reynolds number (Re = ... more The large-scale coherent structures in the wake of a sphere at subcritical Reynolds number (Re = 3700) are examined at different degrees of resolution (filter size) using scale-resolving simulations (SRS) of turbulence. The Partially averaged Navier–Stokes (PANS)-SRS method is employed to compute the wake flow at four levels of resolution marked by unresolved-to-total kinetic energy ratios: fk = 0.5, 0.3, 0.2, 0.1. The results from the four simulations are used to compute one-point statistics, frequency spectra, and spatio-temporal POD (proper orthogonal decomposition) modes. One-point statistics are reasonably well reproduced (in comparison with existing data) at all degrees of resolution. However, the frequency spectra and POD modes exhibit some degree of dependence on the level of resolution. It is demonstrated that accurate depiction of coherent structures is contingent upon adequately resolving key underlying instabilities. The finest resolution PANS simulation (fk = 0.1) is then used to characterize the large-scale coherent structures in the near-wake of the sphere. The main features of this wake, specifically azimuthal mode shapes, modal energy, and frequency content, are compared with experiments and numerical simulations of wakes of other axisymmetric bodies resulting in good agreement. Overall, the study demonstrates that PANS-SRS can capture key one-point statistics and coherent structure features of complex flows in a cost-efficient manner.

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Research paper thumbnail of Partially Averaged Navier–Stokes Formulation of a Two-Layer Turbulence Model

AIAA Journal, 2019

Partially averaged Navier–Stokes (PANS) is a scale-resolving simulation (SRS) approach wherein a ... more Partially averaged Navier–Stokes (PANS) is a scale-resolving simulation (SRS) approach wherein a two-equation closure model is suitably adapted for computing unresolved stresses. The objective of t...

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Research paper thumbnail of Linear analysis of non-local physics in homogeneous turbulent flows

Physics of Fluids, 2019

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Research paper thumbnail of Preliminary Magnetohydrodynamic Simulations of Magnetic Nozzles

We present preliminary results from two computational magnetohydrodynamic (MHD) studies of magnet... more We present preliminary results from two computational magnetohydrodynamic (MHD) studies of magnetic nozzles. The first study models the magnetic nozzle expansion of a plasma into a near vacuum with a resistive MHD solver. Results from resistive MHD simulations suggest that further model development is necessary to replicate experimental results within the desired physical regime. The second study investigates the flow of a magnetically guided plasma into a background plasma using a generalized Ohm’s law MHD solver. Results from the generalized Ohm’s law MHD simulations of the non-expanding jet are obtained using the Magneto-Gas Kinetic Method. Generalized Ohm’s law simulations show bulk rotation of the plasma jet and the formation of helical structures in the velocity and current due to the Hall effect.

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Research paper thumbnail of Unified Gas Kinetic Scheme and Direct Simulation Monte Carlo Computations of High-Speed Lid-Driven Microcavity Flows

Communications in Computational Physics, 2015

Accurate simulations of high-speed rarefied flows present many physical and computational challen... more Accurate simulations of high-speed rarefied flows present many physical and computational challenges. Toward this end, the present work extends the Unified Gas Kinetic Scheme (UGKS) to a wider range of Mach and Knudsen numbers by implementing WENO (Weighted Essentially Non-Oscillatory) interpolation. Then the UGKS is employed to simulate the canonical problem of lid-driven cavity flow at high speeds. Direct Simulation Monte Carlo (DSMC) computations are also performed when appropriate for comparison. The effect of aspect ratio, Knudsen number and Mach number on cavity flow physics is examined leading to important insight.

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Research paper thumbnail of Theory of the lattice Boltzmann method: Two-fluid model for binary mixtures

Physical Review E, 2003

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Research paper thumbnail of Lattice Boltzmann simulations of decaying homogeneous isotropic turbulence

Physical Review E, 2005

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Research paper thumbnail of On the Behavior of Velocity Fluctuations in Rapidly Rotating Flows

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Research paper thumbnail of Scale-resolving simulations of spatially evolving turbulence: Physically consistent inflow specification of unresolved velocity and length-scale profiles

Physical Review Fluids

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Research paper thumbnail of Smart Structures and Materials

Measurement Science and Technology, 1996

ABSTRACT

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Research paper thumbnail of Influence of Knudsen and Mach numbers on Kelvin-Helmholtz instability

Physical Review E

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Research paper thumbnail of Pulsating Flow Past a Square Cylinder: Analysis of Force Coefficient Spectra and Vortex Structure Development

Journal of Fluids Engineering

This study examines the changes in force coefficients and wake flow structures of a square cylind... more This study examines the changes in force coefficients and wake flow structures of a square cylinder subject to pulsating in-flow at different frequencies. The Reynolds number is 200, according to previous literature. Over a range of forcing frequencies, a regime is observed where the shedding frequency scales with the forcing frequency rather than the natural shedding frequency, known as the lock-in phenomenon in literature. The change in spectral character across three frequency regimes—pre-lock-in, lock-in, and post-lock-in—are examined and characterized. During pre-lock-in, the shedding frequency remains equal to the natural shedding frequency. However, the corresponding peak in lift coefficient (CL) power spectral density (PSD) is a single decade larger than that of neighboring minima. This contrasts greatly with subsequent regimes where the amplitudes of the peaks are observed to be substantially larger than the amplitudes of neighboring minima. The onset of lock-in is sharp, a...

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Research paper thumbnail of Magnetic–Internal–Kinetic Energy Interactions in High-Speed Turbulent Magnetohydrodynamic Jets

Journal of Fluids Engineering

The goal of this study is to investigate the interactions between turbulent kinetic, internal, an... more The goal of this study is to investigate the interactions between turbulent kinetic, internal, and magnetic energies in planar magnetohydrodynamic (MHD) jets at different regimes of Mach and Alfvén Mach numbers. Toward this end, temporal simulations of planar MHD jets are performed, using two types of initial fluctuating velocity field: (i) single velocity perturbation mode with a streamwise wavevector and (ii) random, isotropic perturbations over a band of wavevectors. At low Mach numbers, magnetic tension work results in a reversible exchange of energy between fluctuating velocity and magnetic fields. At high Alfvén Mach numbers, this exchange results in the equipartition of turbulent kinetic and magnetic energies. At higher Mach numbers, dilatational kinetic energy is (reversibly) exchanged with internal and magnetic energies, by means of pressure-dilatation and magnetic-pressure-dilatation, respectively. Therefore, at high Mach and Alfvén Mach numbers, dilatational kinetic energ...

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Research paper thumbnail of Nonlinear evolution of perturbations in high Mach number wall-bounded flow: Pressure–dilatation effects

Physics of Fluids, 2020

We characterize the nonlinear evolution of perturbations in a high Mach number Poiseuille flow an... more We characterize the nonlinear evolution of perturbations in a high Mach number Poiseuille flow and contrast the behavior against an equivalent incompressible flow. The focus is on the influence of pressure–dilatation on (i) internal energy evolution, (ii) kinetic–internal energy exchange, and (iii) kinetic energy spectrum evolution. We perform direct numerical simulations of plane Poiseuille flow at different Mach numbers subject to a variety of initial perturbations. In all high-speed cases considered, pressure dilatation leads to energy equipartition between wall-normal velocity fluctuations (dilatational kinetic energy) and pressure fluctuations (a measure of internal energy). However, the effect of pressure–dilatation on the kinetic energy spectral growth can be varied. In cases wherein pressure–dilatation is larger than the turbulent kinetic energy production, spectral growth is considerably slow relative to an equivalent low Mach number case. When pressure–dilatation is smalle...

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Research paper thumbnail of Continuum breakdown in compressible mixing layers

Physical Review E

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Proxy-equation' paradigm - A novel strategy for massively-parallel asynchronous computations

Massively parallel simulations of transport equation systems call for a paradigm change in algori... more Massively parallel simulations of transport equation systems call for a paradigm change in algorithm development to achieve efficient scalability. Traditional approaches require time synchronization of processing elements (PEs) which severely restricts scalability. Relaxing synchronization requirement introduces error and slows down convergence. In this paper, we propose and develop a novel `proxy-equation' concept for a general transport equation that (i) tolerates asynchrony with manageable added error, (ii) preserves convergence order and (iii) scales efficiently on massively parallel machines. The central idea is to modify a priori the transport equation at the PE boundaries to offset asynchrony errors. Proof-of-concept computations are performed using a one-dimensional advection-diffusion equation. The results demonstrate the promise and advantages of the present strategy.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Magneto-Gas Kinetic Method for Nonideal

In this work, the gas-kinetic method (GKM) is enhanced with resistive and Hall magnetohy-drodynam... more In this work, the gas-kinetic method (GKM) is enhanced with resistive and Hall magnetohy-drodynamics (MHD) effects. Known as MGKM (for MHD–GKM), this approach incorporates additional source terms to the momentum and energy conservation equations and solves the magnetic field induction equation. We establish a verification protocol involving numerical solutions to the one-dimensional (1D) shock tube problem and two-dimensional (2D) channel flows. The contributions of ideal, resistive, and Hall effects are examined in isolation and in combination against available analytical and computational results. We also simulate the evolution of a laminar MHD jet subject to an externally applied magnetic field. This configuration is of much importance in the field of plasma pro-pulsion. Results support previous theoretical predictions of jet stretching due to magnetic field influence and azimuthal rotation due to the Hall effect. In summary, MGKM is estab-lished as a promising tool for investiga...

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Towards Reconstruction of Complex Flow Fields Using Unit Flows

Fluids, 2021

Many complex turbulent flows in nature and engineering can be qualitatively regarded as being con... more Many complex turbulent flows in nature and engineering can be qualitatively regarded as being constituted of multiple simpler unit flows. The objective of this work is to characterize the coherent structures in such complex flows as a combination of constituent unitary flow structures for the purpose of reduced-order representation. While turbulence is clearly a non-linear phenomenon, we aim to establish the degree to which the optimally weighted superposition of unitary flow structures can represent the complex flow structures. The rationale for investigating such superposition stems from the fact that the large-scale coherent structures are generated by underlying flow instabilities that may be reasonably described using linear analysis. Clearly, the degree of validity of superposition will depend on the flow under consideration. In this work, we take the first step toward establishing a procedure for investigating superposition. Experimental data of single and triple tandem jets ...

Bookmarks Related papers MentionsView impact

Research paper thumbnail of The Prospect and Benefits of Using the Partial-Averaged Navier-Stokes Method for Engine Flows

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Manufactured Turbulence with Langevin equations

By definition, Manufactured turbulence(MT) is purported to mimic physical turbulence rather than ... more By definition, Manufactured turbulence(MT) is purported to mimic physical turbulence rather than model it. The MT equations are constrained to be simple to solve and provide an inexpensive surrogate to Navier-Stokes based Direct Numerical Simulations (DNS) for use in engineering applications or theoretical analyses. In this article, we investigate one approach in which the linear inviscid aspects of MT are derived from a linear approximation of the Navier-Stokes equations while the non-linear and viscous physics are approximated via stochastic modeling. The ensuing Langevin MT equations are used to compute planar, quadratic turbulent flows. While much work needs to be done, the preliminary results appear promising.

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Research paper thumbnail of Characterization of coherent structures in turbulent wake of a sphere using partially averaged Navier–Stokes (PANS) simulations

Physics of Fluids, 2020

The large-scale coherent structures in the wake of a sphere at subcritical Reynolds number (Re = ... more The large-scale coherent structures in the wake of a sphere at subcritical Reynolds number (Re = 3700) are examined at different degrees of resolution (filter size) using scale-resolving simulations (SRS) of turbulence. The Partially averaged Navier–Stokes (PANS)-SRS method is employed to compute the wake flow at four levels of resolution marked by unresolved-to-total kinetic energy ratios: fk = 0.5, 0.3, 0.2, 0.1. The results from the four simulations are used to compute one-point statistics, frequency spectra, and spatio-temporal POD (proper orthogonal decomposition) modes. One-point statistics are reasonably well reproduced (in comparison with existing data) at all degrees of resolution. However, the frequency spectra and POD modes exhibit some degree of dependence on the level of resolution. It is demonstrated that accurate depiction of coherent structures is contingent upon adequately resolving key underlying instabilities. The finest resolution PANS simulation (fk = 0.1) is then used to characterize the large-scale coherent structures in the near-wake of the sphere. The main features of this wake, specifically azimuthal mode shapes, modal energy, and frequency content, are compared with experiments and numerical simulations of wakes of other axisymmetric bodies resulting in good agreement. Overall, the study demonstrates that PANS-SRS can capture key one-point statistics and coherent structure features of complex flows in a cost-efficient manner.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Partially Averaged Navier–Stokes Formulation of a Two-Layer Turbulence Model

AIAA Journal, 2019

Partially averaged Navier–Stokes (PANS) is a scale-resolving simulation (SRS) approach wherein a ... more Partially averaged Navier–Stokes (PANS) is a scale-resolving simulation (SRS) approach wherein a two-equation closure model is suitably adapted for computing unresolved stresses. The objective of t...

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Research paper thumbnail of Linear analysis of non-local physics in homogeneous turbulent flows

Physics of Fluids, 2019

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Preliminary Magnetohydrodynamic Simulations of Magnetic Nozzles

We present preliminary results from two computational magnetohydrodynamic (MHD) studies of magnet... more We present preliminary results from two computational magnetohydrodynamic (MHD) studies of magnetic nozzles. The first study models the magnetic nozzle expansion of a plasma into a near vacuum with a resistive MHD solver. Results from resistive MHD simulations suggest that further model development is necessary to replicate experimental results within the desired physical regime. The second study investigates the flow of a magnetically guided plasma into a background plasma using a generalized Ohm’s law MHD solver. Results from the generalized Ohm’s law MHD simulations of the non-expanding jet are obtained using the Magneto-Gas Kinetic Method. Generalized Ohm’s law simulations show bulk rotation of the plasma jet and the formation of helical structures in the velocity and current due to the Hall effect.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Unified Gas Kinetic Scheme and Direct Simulation Monte Carlo Computations of High-Speed Lid-Driven Microcavity Flows

Communications in Computational Physics, 2015

Accurate simulations of high-speed rarefied flows present many physical and computational challen... more Accurate simulations of high-speed rarefied flows present many physical and computational challenges. Toward this end, the present work extends the Unified Gas Kinetic Scheme (UGKS) to a wider range of Mach and Knudsen numbers by implementing WENO (Weighted Essentially Non-Oscillatory) interpolation. Then the UGKS is employed to simulate the canonical problem of lid-driven cavity flow at high speeds. Direct Simulation Monte Carlo (DSMC) computations are also performed when appropriate for comparison. The effect of aspect ratio, Knudsen number and Mach number on cavity flow physics is examined leading to important insight.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Theory of the lattice Boltzmann method: Two-fluid model for binary mixtures

Physical Review E, 2003

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Lattice Boltzmann simulations of decaying homogeneous isotropic turbulence

Physical Review E, 2005

Bookmarks Related papers MentionsView impact

Research paper thumbnail of On the Behavior of Velocity Fluctuations in Rapidly Rotating Flows

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Scale-resolving simulations of spatially evolving turbulence: Physically consistent inflow specification of unresolved velocity and length-scale profiles

Physical Review Fluids

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Research paper thumbnail of Smart Structures and Materials

Measurement Science and Technology, 1996

ABSTRACT

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Influence of Knudsen and Mach numbers on Kelvin-Helmholtz instability

Physical Review E

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Pulsating Flow Past a Square Cylinder: Analysis of Force Coefficient Spectra and Vortex Structure Development

Journal of Fluids Engineering

This study examines the changes in force coefficients and wake flow structures of a square cylind... more This study examines the changes in force coefficients and wake flow structures of a square cylinder subject to pulsating in-flow at different frequencies. The Reynolds number is 200, according to previous literature. Over a range of forcing frequencies, a regime is observed where the shedding frequency scales with the forcing frequency rather than the natural shedding frequency, known as the lock-in phenomenon in literature. The change in spectral character across three frequency regimes—pre-lock-in, lock-in, and post-lock-in—are examined and characterized. During pre-lock-in, the shedding frequency remains equal to the natural shedding frequency. However, the corresponding peak in lift coefficient (CL) power spectral density (PSD) is a single decade larger than that of neighboring minima. This contrasts greatly with subsequent regimes where the amplitudes of the peaks are observed to be substantially larger than the amplitudes of neighboring minima. The onset of lock-in is sharp, a...

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Magnetic–Internal–Kinetic Energy Interactions in High-Speed Turbulent Magnetohydrodynamic Jets

Journal of Fluids Engineering

The goal of this study is to investigate the interactions between turbulent kinetic, internal, an... more The goal of this study is to investigate the interactions between turbulent kinetic, internal, and magnetic energies in planar magnetohydrodynamic (MHD) jets at different regimes of Mach and Alfvén Mach numbers. Toward this end, temporal simulations of planar MHD jets are performed, using two types of initial fluctuating velocity field: (i) single velocity perturbation mode with a streamwise wavevector and (ii) random, isotropic perturbations over a band of wavevectors. At low Mach numbers, magnetic tension work results in a reversible exchange of energy between fluctuating velocity and magnetic fields. At high Alfvén Mach numbers, this exchange results in the equipartition of turbulent kinetic and magnetic energies. At higher Mach numbers, dilatational kinetic energy is (reversibly) exchanged with internal and magnetic energies, by means of pressure-dilatation and magnetic-pressure-dilatation, respectively. Therefore, at high Mach and Alfvén Mach numbers, dilatational kinetic energ...

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